On Friday, Secretary of Homeland Security Janet Napolitano resigned to take up a post running California’s university system. With her departure, there are now 15 vacant positions at the top of the department. That suggests it would be a particularly humane moment to shut the whole thing down. The U.S. Department of Homeland Security was a panicked reaction to the Sept. 11 attacks. It owes its continued existence to a vastly exaggerated assessment of the threat of terrorism. The department is also responsible for some of the least cost-effective spending in the U.S. government. It’s time to admit that creating it was a mistake.